igb: add pci device pointer to ring structure

This patch adds a pci device pointer to the ring structure.  The main use of
this pointer is for memory mapping/unmapping of the rings.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>